Over the years, semiconductor devices have been developed in various forms and using a variety of different materials. A more common and conventional semiconductor device uses silicon (Si) as the main foundational material. As will be discussed further below, semiconductor-device research has explored the benefits of materials amenable to liquid-phase crystalline growth such as germanium-based (germanium-including) materials.
In silicon-based semiconductor devices, active devices are typically formed in the surface region of the bulk silicon substrate. In some instances, an epitaxially grown Si layer may be formed over a bulk Si substrate and active devices are formed in the epitaxially grown layer. In such conventional applications, significant capacitance is generally present across a device junction that exists in the bulk silicon substrate or in the overlying epitaxially grown layers. This capacitance tends to slow down the switching speed of circuitry.
One semiconductor application that has been implemented with silicon to reduce the capacitance associated with a bulk silicon junction involves the use of an insulative layer (e.g., oxide) to separate epitaxial silicon from bulk silicon and is commonly referred to as a silicon-on-insulator (SOI) structure. In an SOI structure, the insulator layer greatly reduces the device junction capacitances. The relatively reduced capacitance associated with SOI applications is beneficial for increasing switching speed in switching applications (e.g., transistors), where capacitance delays device switching.
While SOI structures have been found useful in reducing the capacitance typically associated with conventional silicon applications, the epitaxial silicon in the SOI structure exhibits relatively low carrier mobility. Germanium is an example material that can be a desirable alternative to silicon for a variety of applications, largely because germanium exhibits a carrier mobility that is very high relative to that in silicon. For instance, germanium is a promising channel material for MOS-type transistors due to this high carrier mobility. Germanium also has other material properties that differ from silicon, such as a smaller bandgap. These properties facilitate optoelectronic devices and many additional device options. In the past few decades, the use of germanium as well as other materials for integrated circuit applications have been investigated and implemented due to their enhanced qualities, relative to other types of semiconductor materials such as silicon.
The use of semiconducting materials such as germanium-type materials with an implementation similar to SOI (i.e., germanium-on-insulator (GeOI)) would accordingly be useful to achieve relatively low leakage current together with high performance associated with a low-capacitance interface with the insulator layer, similar to that exhibited with SOI.
Single-crystal materials are desirable for use in active regions due to their characteristics relative to, for example, polycrystalline materials. However, single-crystal materials, such as germanium, are difficult to manufacture. In addition, when germanium is grown by epitaxy methods at a seed interface that includes silicon, a lattice mismatch (e.g., about 4%) between the germanium and silicon can result in defects that propagate from this seed interface. This lattice mismatch typically exists between any two different types of crystalline materials. Other approaches to forming single-crystalline materials, such as those using separation by implanted oxygen (SIMOX), wafer bonding, chemical vapor deposition (CVD) epitaxial overgrowth and solid-phase epitaxial growth (SPE) have been relatively complex and difficult to use.
The above and other difficulties have been challenging to the implementation of single-crystal-based materials in a variety of semiconductor applications.